Type Geological formation Country United States | Primary Fluvial Named by Kirk Bryan | |
Area 175 miles (282 km) by 75 miles (121 km) Region Eastern Washington & northwestern Idaho Overlies Columbia River Basalt Group Underlies Columbia River Basalt Group | ||
The Latah Formation is a series of late Miocene lacustrine sedimentary deposits which outcrop in eastern Washington and northwestern Idaho. The lake beds are interbedded with igneous rock of the Columbia River Basalt Group. The formation was originally named from a site in Spokane, Washington by Dr. Kirk Bryan in 1923. Numerous fossil plants and insects have been recovered from the formation and described. When first described the formation was thought to have predated the deposition of the Columbia River Basalts, however further investigation showed them to be interbedded, being laid down in successive events. Potassium-argon dating of the formation returned an age range of 21.3 to 12.1 million years old, indicating an Early to Middle Miocene age range.